The invention relates to a power supply unit for electronic flash, and more particularly, to such unit which is used to feed a main discharge capacitor of an electronic flash.
A power supply unit for an electronic flash is adapted to charge a main discharge capacitor to a given level which is sufficient to cause the emission of flashlight from a flash discharge tube. Such unit operates to maintain the voltage across the capacitor at a substantially constant level. An example of such power supply unit is illustrated in FIG. 1.
Referring to FIG. 1, a power supply unit shown includes a low voltage d.c. source, shown as a battery 1, having its positive terminal connected to the ground through a power switch 2. The negative terminal of the battery 1 is connected to the ground through a series combination of a resistor 3 and a capacitor 4 and is also connected to the emitter of an NPN transistor 6. The transistor has its base connected through a resistor 6a to the collector of a PNP transistor 5 and has its collector connected to one end of a primary coil of an oscillation and step-up transformer 7, the other end of which is connected to the ground. The transistor 5 has its emitter connected to the ground and has its base connected to the junction between the resistor 3 and the capacitor 4 and also to one end of a secondary coil of the transformer 7, the other end of which is connected to the anode of a diode 8. Resistors 3, 6a, capacitor 4, transistors 5, 6, transformer 7 and diode 8 form in combination a DC-DC converter 20. The cathode of the diode 8 is connected to the positive terminal of a main discharge capacitor 9, the negative terminal of which is connected to the ground. The capacitor 9 is shunted by a trigger circuit 10 and also by a flash discharge tube 11.
The trigger circuit 10 includes a series combination of a resistor 12 and synchro contact 13, which combination is connected in parallel with the capacitor 9. The junction between the resistor 12 and the contact 13 is connected through a trigger capacitor 14 to one end of a primary coil of a trigger transformer 15, the other end of which is connected to the ground as is one end of a secondary coil thereof. The other end of the secondary coil is connected to a trigger electrode 16 of the flash discharge tube 11. A circuit is provided which operates to cease the oscillating operation of a converter 20 in response to the detection that the voltage across the capacitor 9 has reached a given level. This circuit comprises a series combination of a resistor 17 and a gaseous discharge tube 18, the combination having one end thereof connected to the positive terminal of the capacitor 9. The circuit also includes an NPN transistor 19 having its base connected to the other end of the series combination. The transistor 19 has its emitter connected to the base of the transistor 5 and its collector connected to the ground.
In operation, when the power switch 2 is closed, the transistors 5 and 6 in the converter 20 are repeatedly turned on and off, thus producing an oscillation. An intermittent current flow occurs through the primary coil of the transformer 7 through a path including the transistor 6, and as a result, a high tension alternating current is induced across the secondary coil of the transformer 7. The secondary voltage is applied through the diode 8 to the main capacitor 9, which therefore is charged and its terminal voltage increases gradually. When the voltage across the capacitor 9 reaches a given level, which is indicated at V.sub.1 in FIG. 2, the gaseous discharge tube 18 discharges, supplying a base current to the transistor 19, thus rendering it conductive. As a consequence, the base-emitter path of the transistor 5 is short-circuited, and this transistor is turned off, thus ceasing the oscillating operation of the converter 20. During the interruption of the oscillation, the charge stored across the capacitor 9 is dissipated in a gradual manner as a result of the base current supplied to the transistor 19 and the self-discharge of the capacitor 9 as the time passes. When the voltage across the capacitor 9 reduces to a level indicated at V.sub.2 in FIG. 2, the gaseous discharge tube 18 ceases to discharge, whereupon the emitter-collector path of the transistor 19 presents a high resistance, allowing the transistor 5 to be turned on to allow the converter 20 to resume oscillation. The described operation is repeated to maintain the voltage across the capacitor 9 substantially constant between the levels V.sub.1 and V.sub.2.
However, in the described power supply unit, it will be noted that the charge stored across the capacitor 9 is dissipated as a base current to the transistor 19, causing an accelerated reduction in the terminal voltage of the capacitor 9. In addition, if the power switch 2 is inadvertently left closed, the charging and discharge process of the capacitor 9 as illustrated in FIG. 2 is repeated, causing the battery 1 to be rapidly exhausted.